Elena Siegman - 115 Lyrics Meaning

So, after analysing this for about...10 minutes, I think I've got it.

Okay, so first off "No one can see me and I've lost all feeling" is referring to Samantha being dead; a ghost. "This house is not my home" Simply put, she lives there, but is not apart of it (relating to the house isn't a home without family bull).

The chorus is pretty straight-forward, so I won't bother explaining.
"They're waiting for the second coming again" I believe is referring to the previous line of everyone dying, and turning into zombies (Second coming)

"Bring me 115" is relating to the Element 115, in which was the source of the Ray Gun, Teleporter, and of course, the creation of the Zombies themselves.

"Left hand of all that is sacred" is, sadly, a biblical reference to the Archangel, Gabriel, the angel of Revelation; the end of the world.

"Pestilence" yet another religious reference to the Horseman of the Apocalypse, also, bugs swarm rotting corpses (ie, Zombies). Also, other Horsemen are present; The blood stain on walls resembling the Grim Reaper (Death), the Zombies feeding on humans (Famine), and killing of things (War)

"Look to the skies" In the Kino Der Toten map on COD: BO, you look to the sky in the alleyway to find a blue light, which shows the location of the Mystery Box, something which players go to for Ray Guns, a bi-product of Element 115.

Again, the Bridge is pretty self-explanatory, so no need to go in-depth.

And the Outro, again, explains itself pretty much.